In a differential dosing scale a continuously operating discharge conveyor discharges a defined dosage or material quantity from the dosing scale, also referred to herein as "weigher". The material quantity or dosage per unit of time which is continuously discharged by a feeder device from a container of the weigher is determined by deriving the measured values of the total weight with respect to time. However, the respective signal or value that constitutes the weight rate comprises in addition to the actual weight rate signal component a number of signal components that are caused by interfering or disturbing influences. For example, a nonhomogeneous bulk material flow or a nonlinear response characteristic of the feeder device can cause output disturbances which must be sensed or measured and indicated or these signal components must be eliminated by a closed loop control to avoid falsifying the measured result.
Incidentally, in this disclosure the terms signal or signals are used as synonymous terms for value or values.
In addition to the above mentioned interfering signal components there is another group of interferences or disturbances which may have an adverse effect on the measured weight signal, however which does not cause variations in the output material flow rate. This other group of interferences are referred to herein as pure measuring interferences and include, for example, vibrations in the building where the weigher is located. Such vibrations are frequently caused by heavy trucks or trains passing by the building. Further, to this group of pure measuring interferences also belong interferences caused by an object inadvertently deposited on the weigher, contact of the weigher container by an operator, and similar influences. Even wind influences to which the weigher container may be exposed can cause these pure measuring interferences. These pure measuring interferences adversely affect the measured weight signal even if they do not vary the output flow. Differential dosage scales usually must meet very high precision requirements in industrial processes. Thus, it is necessary that these pure measuring interferences are differentiated from other interferences that vary the material flow. Further, signals representing these pure measuring interferences must be screened out or filtered out from the display of the dosage values and, if applicable, from a closed loop control circuit of the bulk material feeder device forming part of the weigher.
Two kinds of interferences occur, both of which affect the measured signal W(t) and as a consequence also the time W'(t) derivative (feed rate). Feed rate interference z.sub.A (t) are actual interferences in the feed rate A(t), which have to be corrected by the controller. Pure measurement interferences z.sub.M (t) occur only in the measured value W(t) and not in the feed rate. The controller must not react to such interferences, in order not to corrupt the actual feed rate A(t). The totalized amount of material discharged by the distribution means M.sub.total is obtained by integrating the measured feed rate W'(t). Actual variations of the rate A(t) caused by interferences z.sub.A (t) have to be accounted for during the calculation of the totalized amount. Pure measurement interference z.sub.M (t), however, would corrupt the determination of the totalized amount and have to be eliminated.
European Patent Publication EPO 291,553 and U.S. Pat. No. 4,977,526 (Jost et al.), issued on Dec. 11, 1990, disclose a method for screening out interfering signals occurring in a differential dosing scale. The known method operates by producing estimated values for the future based on values measured in the past. For this purpose, the weight signal is sampled at short time intervals on a continuous basis. The sampled signals are supplied to an evaluating circuit which calculates the difference between the instantaneously measured weight signal and the previously ascertained estimated value.
If the difference between the estimated value and the actually measured weight value exceeds a fixed tolerance range or threshold, a pure measuring interference is ascertained and the estimated value is used as the basis of a closed loop control. Additionally, the estimated value is also displayed so that an operator may take notice. In the known method of filtering or screening out interferences only those pure measuring interferences are ascertained which exceed the threshold value or tolerance range between the estimated value and the respective measured weight value. However, in the known method interferences that are within the tolerance range or which establish themselves slowly in time, are not ascertained. As a result, such falsifying disturbances or interferences are not screened out. Due to the fixed threshold, the known method has little flexibility.